The present invention relates to a method for wet-gunning a dense, castable refractory composition to construct linings of vessels for molten metals such as ladles, tundishes and troughs, etc.
Lately, castable refractories with easy installation have been more and more replaced for bricks as materials for linings of vessels for molten metals, as the castable refractories have been provided with improved durability. However, casting methods are still disadvantageous in terms of time and labor needed for placement of molds in casting. In these respects, gunning methods are advantageous in reduced time and labor requirements and in versatility of repair, because these methods do not require the formation of molds and can quickly make partial repair. Accordingly, the gunning methods have been finding more and more applications.
Most widely used is a dry-gunning method. However, refractory layers formed by the dry-gunning method are poor in durability and suffer from rebound loss, poor working environment due to dusts, etc. To overcome the problems of the dry-gunning methods, developments have thus been made recently to provide various gunning methods such as a semi-dry gunning method, a wet-gunning method, etc., and refractory materials therefor.
In the semi-dry gunning method, the gunning refractory material is premixed with part of necessary water by a mixer, pneumatically conveyed to a gunning nozzle of a dry-gunning machine, mixed with the remaining water, or with a solution or a suspension containing a hardening agent in the gunning nozzle (or before reaching the gunning nozzle), and then gunned through the nozzle. The semi-dry gunning methods are exemplified in Japanese Patent Laid-Open No. 61-111973, and Japanese Patent Publication Nos. 2-27308, 6-17273, 5-63437 and 5-21866, etc.
Japanese Patent Laid-Open No. 61-111973 discloses the combination of a hardening accelerator and sodium silicate (binder), and Japanese Patent Publication Nos. 2-27308, 6-17273 disclose low-cement castables. Also, both of Japanese Patent Publication Nos. 5-63437 and 5-21866, owned by the same applicant, disclose castable refractory materials containing ultrafine powder and dispersing agents, which are not hardened at room temperature. Specifically, in the former reference a refractory clay is used as indispensable ultrafine powder and solidified by Ca(OH).sub.2, sodium silicate or sodium aluminate introduced at a nozzle to improve the resistance to dry-explosive spalling. In the latter reference, a refractory composition to which a moisture retention agent is added is premixed with water in an amount of 1/5-3/4 of the normally required amount in a factory to provide a wet refractory material, which is then solidified by sodium silicate, sodium aluminate or colloidal silica introduced at a nozzle.
Though improvements were achieved to some extent in these semi-dry gunning methods in reducing dust generation and rebound loss, these semi-dry gunning methods still provide the formed refractory layers with poor adhesion, homogeneity and density, since the refractory materials should be mixed with water in a gunning nozzle very quickly, resulting in poor mixing at an unstable water ratio.
The wet-gunning methods typically include the following two methods:
(1) Gunning refractories supplied in a dry state are premixed with all of water required for gunning by a mixer at a construction site, conveyed to a nozzle and then gunned. This method may be called "an in-situ premixing, gunning method." PA0 (2) Gunning refractories premixed with all of water required for gunning in a factory are supplied to a nozzle and gunned therethrough. The gunning refractories may be called "factory-premixed water-containing refractory materials," and this gunning method may be called "a factory-premixing, gunning method."
In the wet-gunning methods, it is possible to add a small amount of an aqueous solution of a hardening agent or a hardening-adjusting agent to the gunning refractories in a nozzle. The wet-gunning methods are classified into a gunning machine method and a pumping method depending on conveying means of gunning materials. In the case of the pumping method, compressed air is introduced into the gunning nozzle to spray gunning materials. The present invention belongs to the latter category. Depending on the amount of water added, there are provided gunning materials with various workabilities ranging from a plastic level to a slurry level.
These wet-gunning methods are exemplified in Japanese Patent Publication Nos. 57-7350, 62-21753, 2-33665, 2-1795, etc. Because the gunning materials disclosed in Japanese Patent Publication No. 57-7350 are in the form of a slurry having a water content of 10-20%, they are not expected to be formed into dense refractory layers suitable for linings of vessels for molten metals. In the latter three methods, the gunning refractory materials are supplied in the form of a moist premix containing water or an aqueous solution, making it possible for them to be produced and stored in factories. These wet-gunning methods contribute to the reduction of labor and the improvement of working environment because blending is not needed at the gunning site.
However, these conventional methods do not utilize the dispersion action of ultrafine refractory powder to reduce the amount of water added to the castable refractory compositions. Also, because particles contained in the gunning materials have maximum particle sizes of 4 mm or less, they are still not different from gunning technologies of the previous generation. Accordingly, the gunned refractory layers are insufficient in denseness and thus poor in durability, as compared with those produced by casting methods of dense, castable refractory materials.